The present invention is directed to a wireless communication system using a spread-spectrum communication protocol and, in particular, to such a wireless communication system having a master unit sending polling signals to a plurality of slave units over the at least one wireless communication link to retrieve data from the slave units. The invention may be useful with a wireless response unit which retrieves response data being entered in response units by a user, in a remote data collection system useful to retrieve sensor data from remote sensors, as well as other applications in which data must be wirelessly retrieved at a master unit from a series of slave units.
Spread spectrum protocol is useful in wireless communications. By distributing wireless communication over a large bandwidth, a higher transmission power may be used within government regulations without requiring the payment of licensing fees. One form of spread spectrum is frequency hopping in which the spectrum is divided into a number of frequency channels. The transmission hops between the various channels with substantially equal transmission time in all of the channels being utilized. In order for the transmitter and receiver to operate at the same frequency, it is necessary to provide a technique for coordinating their hopping. This may be accomplished by providing pseudorandom frequency hop tables in the transmitting and receiving units to coordinate the hopping.
While known spread spectrum frequency hopping protocols are useful for certain applications, they have not been proposed for polling based systems, especially ones having a large number of slave units. When a unit is placed into operation, it must become synchronized with the polling sequence. In prior communication systems, such as cordless telephones, this may be accomplished by the receiving unit listening in on a particular channel until it receives a transmission and thereafter following the hopping of the transmitter unit with the assistance of the frequency-hopping table. However, in polling-based systems, the interval between transmissions from the master unit may be sufficiently long that an inordinate amount of time may pass before a transmission occurs on a particular channel.